Stainless steel and aluminum cylinders have long been used for the provision of gases where high purity is a requirement. Stainless steel cylinders, however, are extremely expensive and are difficult to manufacture. Aluminum cylinders provide a clean inert inner surface, but they lack the capacity of carbon steel or stainless steel cylinders and they are associated with cracking where the aluminum is drawn down to provide the cylinder neck.
The present invention utilizes an interior surface finishing process for carbon steel cylinders to provide the benefits of the present invention with respect to very low particulate generation and entrapment properties. In the first step, the interior surface of the cylinder is honed to provide a relatively smooth surface. The interior surface of the cylinder is then electropolished to provide unique surface chemistry and the properties of the invention with respect to cleanliness and low particulate generation and entrapment. In an important embodiment, a vacuum baking step can then be used to eliminate adsorbed impurities and moisture.
It is well known to treat the surface of various metals by electropolishing. U.S. Pat. No. 3,919,061 to Jumer is directed to a method and apparatus for electropolishing or chemical polishing of the entire inner surface of a large cylindrical vessel having at least one closed end. U.S. Pat. No. 3,682,799 to Jumer is also directed to a method for fabricating cylindrical vessels having domed or necked ends wherein the interior surfaces of the vessel including the access ports are provided with an electropolished surface.
U.S. Pat. No. 3,795,597 to Katz, et al. is directed to a method for producing an ultraclean, bright surface on titanium. The method involves treating the titanium surface with a solution of methyl alcohol, sulfuric acid and hydrochloric acid and through which is passed an electric current.
A method for transferring very pure ultimate use gas from a cylinder to a user apparatus through a tube fitted with valves and other distributing and control elements is described in U.S. Pat. No. 3,880,681 to Sifre, et al. In the method, the tubing, valves and other elements are subjected before the transferring and before assembly as a flow line to chemical etching by aqueous solutions of volatile noncontaminating acids, then to decontamination by extensive sweeping with a very pure hot gas to volatilize the acids. The ultimate use gas is subsequently transferred after assembly and additional sweeping by pure hot gas through the decontaminated tubing, valves and other elements to the ultimate user apparatus.
An apparatus for internally electropolishing tubes is described in U.S. Pat. No. 4,705,611 to Grimes. In the apparatus, a plurality of elongated tubes are horizontally supported and rotatably driven about their axes. An outlet fitting including an end dam permits rotation of the tube outlet end and allows escape of gases from the upper portion of the tube to permit overflow of electrolyte liquid thereover.
U.S. Pat. No. 2,412,186 to Whitehouse describes a method of electrolytic polishing to provide a bright surface on the interior of stainless steel tubes.
U.S. Pat. No. 3,405,049 to Czubak is directed to a rotary boring tool including a plurality of cutting tools for performing different cutting operations on a single tool spindle. The rotary boring tool is used for both rough cutting and honing wherein a generally cylindrical bore is sized and finished to close tolerances. The bore is first enlarged or roughed out slightly undersized by a roughing cutter and a finishing cutter designed to remove a relatively small amount of stock is used to accurately size the work piece bore. A honing tool is then used to finish the bore to a high degree of precision and impart a desired surface finish to the bore wall. The Czubak patent describes a single tool which can be used for all of these operations. The Czubak patent indicates that the honing tool assembly may incorporate the use of an electrolytic assist in honing the cylindrical bore by placing contoured electroplates connected to a source of electric current which are disposed in spaced circumferential relation to a spindle. When using the electrolytic assist, fluid is dispersed through the spindle to the workpiece bore through fluid distributing openings.
While various means and apparatus for treating the interior surface of cylindrical objects are known, it would be desirable to provide a means for providing a very highly polished surface on the interior of carbon steel cylinders intended for use in containing a user gas of very high purity under high pressure and providing the user gas for specialized operations, such as semiconductor manufacturing processes. As indicated, the use of stainless steel cylinders for such purposes is very expensive and the use of aluminum cylinders is not practical due to structural problems incurred while providing the necessary tapered neck in the end of the cylinder.
Accordingly, it is a principal object of the present invention to provide a method for distributing a substantially particulate free gas wherein the cylinder containing the gas is provided with a smooth, inert surface of unique chemical composition.
It is another object of the present invention to provide a method for dispensing a highly pressurized gas from a carbon steel cylinder wherein the gas is dispensed in a substantially particulate and contaminant free form.
These and other objects of the present invention will become more apparent from the following detailed description and the accompanying drawings.